Several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form, because it occupies a smaller volume and does not need to be stored at a high pressure.
U.S. Pat. No. 3,763,658 describes a refrigeration system and method for liquefying a feed stream by subjecting the feed stream to heat exchange with two refrigerants. After use, the second refrigerant is compressed in two compressor stages, but even with an intercooler and aftercooler, it requires passing through two propane exchangers before achieving at least partial condensation prior to a phase separator. This requires substantial condensing duty in the propane exchangers, taking away some of their cooling ability for cooling other streams.
It is an object of the present invention to improve the efficiency of a cooling process and apparatus. It is another object of the invention to increase the capacity of a hycrocarbon process.